1. Field of the Invention
The present invention relates to a semiconductor resistor element that can be suitably used as a load resistor for static memory cells.
2. Prior Art
Static random access memories necessitate load resistors for supplying electric charge to assure static property. The load resistors constitute a memory cell together with a flip-flop circuit formed by a pair of transistors and in the flip-flop circuit one of the transistors is usually conductive and the other one is nonconductive. The load resistors supply a current to render the transistors conductive or nonconductive, and also work to maintain the transistors conductive. To minimize the power consumption of memories, however, the resistors should have a high resistance.
In recent years, there have been proposed memory cells which feature a reduced power requirement. For example, a power-supply current which had so far been required on the order of 50 to 100 mA has now been reduced to 10 to 20 mA when the memory is in the stand-by mode. With the memory of this type, the flow of current is limited when the peripheral circuits are under the stand-by condition in order to reduce the consumption of electric power. However, it is also important to reduce the consumption of electric power by the memory cells. The effort to reduce the consumption of power by the peripheral circuits becomes meaningless if heavy current flows through the memory cells. In particular, the number of memory cells increases with the increase in the memory capacity, while the peripheral circuits are not so increased. It is, therefore, important to construct memory cells which consume less electric power.
From the viewpoint of reducing the consumption of electric power and increasing the degree of integration, in recent years, a high-resistance polycrystalline silicon film has more often been used as the load. This is disclosed, for example, in U.S. Patent specification No. 4,110,776. This silicon film exhibits a greatly varying resistance depending upon the concentration of impurities; a high resistance can be easily obtained if the concentration of impurities is decreased. The concentration of impurities can be easily and accurately adjusted by ion implantation. However, the resistance of a polycrystalline silicon resistor has a very great temperature gradient. At an ordinary temperature, for example, the electric current per cell will be from 1 to 100 nA. At high temperatures, however, the current increases by about ten times, i.e., the current of 10 to 1000 nA flows. This presents a serious problem when the memory has large capacities. If it is attempted to increase the load resistance such that the consumption of electric power remains sufficiently small even at high temperatures, the operation becomes defective at an ordinary temperature. For instance, the operation of the flip-flop circuit becomes slow, and it becomes difficult to sufficiently supply a leakage current across the source and drain of the transistor. Accordingly, the potential at the node changes, and it becomes impossible to maintain the transistors conductive or nonconductive.